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“Unprotected” Pt nanocrystals were modified with triphenylphosphine (PPh3), octadecylamine (ODA), poly(vinylpyrrolidone) (PVP), poly(vinyl alcohol) (PVA), and dodecanethiol (DT) to investigate the effect of protective agents on the intrinsic catalytic property of Pt nanocrystals. By evaluating the catalytic performance of these model catalysts for the hydrogenation of para‐chloronitrobenzene (p‐CNB), it was found that direct or indirect interaction between nanocrystals and protective agents imposed a great impact on the catalytic performance of the nanocrystals. Protective agents with different electron‐donating ability (PPh3, ODA, PVP, and PVA) directly altered surface electronic state of Pt nanocrystals to bring the surface Pt atoms into an electron‐rich state, which would exert influence on the hydrogenation course by changing the adsorption and the reactivity of reactant, intermediates, and products. In contrast, DT exerted an indirect influence on the Pt nanocrystals. The coordinated Pt atoms were oxidized by DT to generate cationic Pt species on the surface of nanocrystals, and the cationic species would simultaneously improve the hydrogenation rate and selectivity to para‐chloroaniline by polarizing the N=O bond in the −NO2 group of p‐CNB and altering the electronic state of Pt nanocrystals, respectively. This work provided further insights into nanocatalysis, which is helpful for further design and application of highly efficient nanocrystal catalysts.

Unprotected" metal and alloy nanoclusters prepared using the alkaline-ethylene glycol method (AEGM), stabilized by adsorbed solvent molecules and simple ions, have been widely applied in the development of high-performance heterogeneous catalysts and the exploration of the effects of metal particle size and composition, surface ligands of support, and modifiers on the catalytic properties of heterogeneous catalysts. The formation process and mechanism of such unprotected metal nanoclusters need to be further investigated. In this study, the formation process and mechanism of unprotected Pt and Ru nanoclusters prepared with AEGM were investigated by in situ quick Xray absorption fine spectroscopy (QXAFS), in situ ultraviolet-visible (UV-Vis) absorption spectroscopy, transmission electron microscopy, and dynamic light scattering. It was discovered that during the formation of unprotected Pt nanoclusters, a portion of Pt(IV) species was reduced to Pt(II) species at room temperature. With increasing temperature, Cl − coordinated to Pt ions was gradually replaced with OH − to form intermediate platinum complexes, which further condensated to form colloidal nanoparticles. Obvious scattering signals of the colloidal nanoparticles could be observed in the UV-Vis absorption spectra of the reaction system before the formation of Pt-Pt bonds, as revealed by QXAFS measurements. In situ QXAFS analysis revealed that Pt nanoclusters were derived from the reduction of Pt oxide nanoparticles. The average particle size of the nanoparticles obtained by heating the reaction mixture for 15 min at 80 C was 3.7 nm. High resolution transmission electron microscopy (HRTEM) images showed that the spacing between the crystal planes of the nanoparticles was 0.249 nm, indicating that the intermediate nanoparticles were platinum oxide. As the reaction proceeded, the average size of the nanoparticles decreased to 2.4 nm, and two types of nanoparticles were observed having different contrasts, corresponding to Pt metal nanoclusters standing on the intermediate metal oxide nanoparticles as confirmed by HRTEM images. When the reaction time was further extended, the average size of nanoparticles decreased to 1.4 nm, and the observed lattice spacing of the nanoparticles was the same as that of Pt( 111) crystal plane at 0.227 nm, indicating that the final products were Pt metal nanoclusters. In general, when metal oxides are reduced to metal nanoclusters, the density of the nanoparticles will increase, whereas the volume will decrease. Moreover, as shown in this study, the formation of multiple small metal nanoclusters standing on one metal oxide nanoparticle was also observed in TEM photographs. Thus, compared with the size of the initial nanoparticles, the average size of the final metal nanoclusters was significantly reduced. On the other hand, during the formation of unprotected Ru metal nanoclusters, Cl − in RuCl3 was first replaced with OH − to form Ru(OH) 6 3− , which further condensated to form Ru oxide nanoparticles, and unprotected Ru met...